Measurement method and apparatus of an external digital camera imager assembly

ABSTRACT

A method for determining whether an imager assembly outside of a camera body meets predetermined focus specifications, wherein the imager assembly includes an image sensor and a camera mounting plate having reference features adapted to cooperate with alignment features in the camera body to locate the image sensor at a predetermined focal plane, including the steps of: mounting the imager assembly onto an imager mounting apparatus having equivalent alignment features; and utilizing low-coherence light interferometry to determine whether the image sensor will meet predetermined focus specifications when mounted in a camera body.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This is a divisional of application Ser. No. 09/697,808, filedOct. 27, 2000 (allowed, Sep. 10, 2002).

FIELD OF THE INVENTION

[0002] The present invention relates generally to opticalinterferometry. More particularly, the present invention relates to amethod and apparatus for determining the location of an imager planewith respect to a camera-mounting plane of an imager assembly.

BACKGROUND OF THE INVENTION

[0003] In a conventional digital camera, an image beam is directedthrough a lens and onto an imager or image sensor comprised of an arrayof sensing elements, for example a Charge Coupled Device (CCD). In orderto provide a focused image, the lens and the imager need to be properlypositioned, relative to each other, within the digital camera.

[0004] The steps in a CCD based imager manufacturing process are asfollows. Multiple CCD imager arrays are processed together on a singlesilicon wafer. Imager dies, composed of a single CCD imager array, arediced from the wafer and positioned and glued into specially designedpackages. A flat transparent plate called the imager cover glass is thenglued into the specially designed package at a location that is offsetfrom the imager die to hermetically seal the specially designed package.This hermetically sealed package containing the imager die is thenmounted into a camera-mounting plate that includes a reference plane tofacilitate proper mounting into the camera. The camera itself willinclude a camera reference plane to receive the camera mounting platefrom the imager package. In a film camera, film rails usually define thecamera reference plane. Optionally, the package can include thecamera-mounting plate and reference plane, which would eliminate thislast step of mounting the hermetically sealed package into a cameramounting plate.

[0005] In order to ensure that the CCD is positioned properly in thecamera, the location of the CCD needs to be determined. Such a locationcan be determined relative to a reference surface or reference plane.

[0006] A Coordinate Measuring Machine (CMM) is an example of anapparatus employed to determine the location of an object relative to areference plane. Conventionally, the object is retained in a suitableholder on an optical bench. In one method to determine the location ofan object, three points on a reference plane, approximately 120 degreesapart, are measured to define the reference plane; the coordinates ofthe three points are tracked in the x, y and z directions. A point onthe object is then measured relative to the reference plane, and thedistance from the reference plane is calculated. Conventional CMMs havecontact probes for intimately contacting each of the points defining thereference plane and the object, such as those described in U.S. Pat. No.5,428,446 issued Jun. 27, 1995 to Ziegart et al. entitled MeasurementInstrument with Interferometer and Method, U.S. Pat. No. 5,446,545issued Aug. 29, 1995 to Taylor entitled Method of and Apparatus forCalibrating Machines Including a Measuring Probe and a MeasuringApparatus, and U.S. Pat. No. 4,929,082 issued May 29, 1990 to Webberentitled Laser Linear Distance Measurement System and Apparatus. Thesereferences include interferometers that monitor the displacement of themachine axes. In contrast, non-contacting methods, such as opticaltriangulation, are described in U.S. Pat. No. 4,373,804 issued Feb. 15,1983 to Pryor et al entitled Method and Apparatus for Electro-OpticallyDetermining the Dimension, Location and Attitude of Objects, and U.S.Pat. No. 5,510,625 issued Apr. 23, 1996 to Pryor et al. entitled Methodand Apparatus for Electro Optically Determining the Dimension, Locationand Attitude of Objects.

[0007] Another technology known as low-coherence light interferometryhas also been used to measure physical properties of an object. U.S.Pat. No. 5,659,392 issued Aug. 19, 1997 to Marcus et al. entitledAssociated Dual Interferometric Measurement Apparatus for Determining aPhysical Property of an Object, and U.S. Pat. No. 5,596,409 issued Jan.21, 1997 to Marcus et al. entitled Associated Dual InterferometricMeasurement Method for Determining a Physical Property of an Object,disclose an associated dual interferometric apparatus and method formeasuring physical properties of an object, such as thickness, groupindex of refraction, and distance to a surface. U.S. Pat. No. 5,757,485issued May 26, 1998 to Marcus et al. entitled Digital Camera ImageSensor Positioning Method Including a Non-Coherent Interferometer, andU.S. Pat. No. 5,757,486 issued May 26, 1998 to Marcus et al. entitledDigital Camera Image Sensor Positioning Apparatus Including aNon-Coherent Light Interferometer, disclose a digital camera imagesensor positioning apparatus and method which includes a low-coherencelight interferometer. The apparatus and method include a removableoptical probe assembly mounted to a digital camera. The low-coherencelight interferometer is in communication with the optical probe assemblyto determine a depth of an image sensor residing within a digitalcamera, relative to a reference surface. U.S. Pat. No. 6,075,601 issuedJun. 13, 2000 to Marcus et al. entitled Optical Probe CalibrationApparatus and Method describes an optical probe calibration apparatusused for calibrating the optical probes used in U.S. Pat. Nos. 5,757,485and 5,757,486 referenced above. These three aforementioned U.S. patentsrequired that the optical probe be mounted in the camera body in orderto determine the location of the imager sensor with respect to thecamera reference surface.

[0008] Heretofore, a skilled operator was required to install the imagerin the camera and subsequently assemble the camera before finding out ifthe imager was properly focused. Several steps were required, includingsecuring the imager with 3 or 4 screws onto the camera-mounting plane,and inserting a measurement optical probe into the camera body andlocking the probe into the lens flange-mounting ring before ameasurement could be initiated. Before mounting the measurement opticalprobe into the camera body, the camera electronics needed to be turnedon and the electronic shutter needed to be opened. Full camera assemblyand substantial skilled operator intervention were required before anassessment of imager focus could be made. If the imager was out offocus, the camera had to be disassembled and the imager replaced. Inorder to calibrate the measurement optical probe, an externalcalibration fixture was also required. The distance from thecamera-mounting ring to the reference surface in the externalcalibration fixture is better suited for measurement with an externaltechnique, such as provided by a CMM machine.

[0009] While internal apparatus and methods may have achieved a certainlevel of success, the internal apparatus is not readily transportablenor simple to use. Further, the methods are time consuming and quiteoften are dependent on the skill of the operator.

[0010] Accordingly, a need continues to exist for an apparatus andmethod for determining the position of an image sensor in a digitalcamera. Furthermore, there is a need to properly predict the position ofan image sensor before permanently physically mounting the image sensorinside the digital camera. The apparatus needs to be robust,transportable and simple to use. The method must be fast, provideobjective results independent of the operator, and provide accurate andconsistent results.

SUMMARY OF THE INVENTION

[0011] The need is met according to the present invention by providing amethod for determining whether an imager assembly outside of a camerabody meets predetermined focus specifications, wherein the imagerassembly includes an image sensor and a camera mounting plate havingreference features adapted to cooperate with alignment features in thecamera body to locate the image sensor at a predetermined focal plane,including the steps of: mounting the imager assembly onto an imagermounting apparatus having equivalent alignment features, and utilizinglow-coherence light interferometry to determine whether the image sensorwill meet predetermined focus specifications when mounted in a camerabody.

[0012] The present invention also provides an imager mounting apparatusto receive an imager assembly in a predetermined orientation fordetermining whether an imager assembly outside of a camera body meetspredetermined focus specifications, including: an optical probe with apellicle reference surface; a camera body mounting equivalent withequivalent alignment features for receiving and aligning the imagerassembly in a predefined orientation; and a plurality of clamps to lockin the predetermined orientation.

[0013] The present invention also provides an interferometric-basedmeasurement system for determining whether an imager assembly outside ofa camera body meets predetermined focus specifications, including: a lowcoherence light interferometer; an imager mounting apparatus includingan optical probe having an optical probe chuck; an optical fiber cablefor coupling light from the interferometer to the optical probe chuck;and a computer for processing data collected by the interferometer,wherein the data is used to determine whether the imager assembly meetspredetermined focus specifications.

[0014] The present invention also provides a method for calibrating anabsolute distance to a reference surface for determining the position ofan imager plane relative to an image sensor camera-mounting referenceplane in an imager assembly, including the steps of: mounting a flatreference plate onto an imager mounting reference surface; and utilizinglow coherence light interferometry to determine the distance between theimager mounting reference surface and a pellicle reference surface(known as PP′) of the imager mounting apparatus.

[0015] The present invention also provides a method for determining aposition of an imager plane relative to an image sensor camera-mountingplane in an imager assembly, including the steps of: temporarilymounting the imager assembly onto an imager mounting apparatus having animager mounting reference surface such that the imager sensorcamera-mounting reference plane and the imager mounting referencesurface are in intimate contact; wherein the imager mounting apparatusincludes an optical probe with a pellicle reference surface in apredetermined orientation with respect to the imager mounting referencesurface such that the pellicle reference surface is disposed at a firstdepth relative to the imager mounting-reference surface; utilizinglow-coherence light interferometry to determine: (i) a second depth fromthe pellicle reference surface to a front surface of the opticallytransparent plate, (ii) an optical thickness of the imager cover glass,and (iii) a third depth from a back surface of the imager cover glass tothe imager plane; and calculating the optical position of the imagerplane relative to the imager sensor camera-mounting reference plane.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1A shows a prior art schematic back view of a camera bodyincluding a camera mounting reference surface;

[0017]FIG. 1B shows a prior art schematic side view of a camera bodywith a camera mounting reference surface and a lens flange mounting ringfor attaching a lens to the camera body;

[0018]FIG. 2A shows a prior art plan view of an example camera mountingplate;

[0019]FIG. 2B shows a prior art cross-section view of an imager assemblyincluding an image sensor and parts of a camera body to indicate theorientation of imager mounting into a camera body;

[0020]FIG. 3 shows a schematic of an example measurement apparatus;

[0021]FIG. 4A shows a schematic of an example clamp table assembly;

[0022]FIG. 4B shows a cross-section view of an example clamp tableassembly in relation to the imager assembly;

[0023]FIGS. 5A and 5B show relational measurement geometry and thecorresponding parameters which are measured utilizing one embodiment ofthe present invention; and

[0024]FIG. 6 shows an example of raw interferometer measurement dataobtained when measuring an imager assembly mounted into the imagermounting apparatus.

DETAILED DESCRIPTION OF THE INVENTION

[0025] A stationary probe apparatus, referred herein as an imagermounting apparatus, has been developed which includes an optical probepermanently mounted at a constant distance from a reference planedesigned to mimic the function of the imager mounting plane in a digitalcamera. The optical probe has a pellicle reference plane built in to itwhich is used as a reference surface to calculate distances. Preferably,the pellicle reference plane is the surface of a thick, glass, opticalflat that faces the imager assembly in the probe mount. This allows oneto assess the imager focus location with respect to an ideal focus,without the need of inserting the optical probe into a camera body. Thestationary fixture, with the permanently mounted optical probe, alsosecures the imager mounting plate in place with a simple clamping means,thus eliminating the need to use screws which add to assembly time. Inorder to calibrate the apparatus an optically flat plate is installedinto the same apparatus to determine the distance from the optical probepellicle reference plane to the plane in the same apparatus that theimager mounting plate is clamped to. Thus, no external apparatus isneeded to calibrate the optical probe.

[0026]FIG. 1A shows a prior art schematic back view of a camera body 30with a camera mounting reference structure 35 with a camera referenceplane 32 for mounting an imager assembly (not shown). In a film camerathe camera reference structure 35 ordinarily includes a pair of camerafilm rails. Also shown in FIG. 1A are the camera threaded holes 36 andthe camera alignment pin receiver holes 34 in the camera referencestructure 35 for mounting the imager assembly. Preferably, one of thecamera alignment pin receiver holes 34 is slotted in order to facilitatemounting of an imager assembly to the camera body.

[0027]FIG. 1B shows a prior art schematic side view of a camera body 30with a camera mounting reference structure 35 with camera referenceplane 32 and a lens flange mounting ring 31 for mounting camera lensesto the camera body. The distance from the lens flange mounting ring 31to the camera reference plane 32 is defined as LR.

[0028] In order to properly focus an imager when mounted into a camerabody it must be located at a defined distance from the lens flangemounting ring within a design tolerance. In the manufacture of precisioncameras such as SLR cameras the distance LR is tightly controlled sothat proper focus can be assessed by determining the distance from thecamera reference plane 32 to the position of an imager die in an imagerassembly.

[0029]FIG. 2A shows a plan view of a prior art camera mounting plate 20disclosing the reference features adapted to cooperate with alignmentfeatures in the camera 30 of FIGS. 1A and 1B, and that enablespositioning an image sensor 12 (shown in FIG. 2B) at a predeterminedfocal plane once mounted inside the camera 30. Usually the predeterminedfocal plane is measured with respect to the lens flange-mounting ring 31(shown in FIG. 1B) of the camera body 30. FIG. 2A shows a cameramounting plate 20, mounting holes 26, alignment pins 24 and an imagesensor cutout 28.

[0030]FIG. 2B shows a prior art cross-section view of an imager assembly10 including an image sensor 12 and parts of a camera body 30 toindicate the orientation of imager mounting into a camera body. Thecross-section view shown in FIG. 2B is that shown by the dashed line inFIG. 2A and labeled 2B. An imager assembly 10, shown schematically inFIG. 2B, includes an image sensor 12 mounted to a camera-mounting plate20 which includes reference features adapted to cooperate with alignmentfeatures in the camera 30 which facilitate locating the image sensor 12at a predetermined focal plane. These reference features include animage sensor camera-mounting reference plane 22, alignment pins 24 and aplurality of mounting holes 26 to facilitate proper mounting into thecamera 30. During assembly the image sensor 12 is attached to the cameramounting plate 20 by bonding means 21. The lens flange-mounting ring 31is also shown in FIG. 2B to show orientation of the camera body.

[0031] Also shown in FIG. 2B, is the camera reference plane 32 that isaligned with the image sensor camera mounting plate reference plane 22during the imager assembly's 10 installation into the camera 30.Alignment pins 24 are installed in camera alignment pin receiver holes34 which cause mounting holes 26 to automatically align with camerathreaded holes 36. The imager assembly 10 is secured to the camera 30with screws (not shown) placed in the camera mounting plate 20 whichpass through mounting holes 26 and are threaded into the camera threadedholes 36.

[0032] The image sensor 12 includes an imager die 13 with an imagerplane 14, offset from an optically transparent imager cover glass 16with front surface 25 and back surface 11 defining an imager gap 18between the imager plane 14 and the back surface of the imager coverglass 16. The imager cover glass 16 can be mounted in an imager package19 with a hermetic seal at the cover glass bond perimeter 23 around theimager die 13. The image sensor 12 also includes imager electricalconnections 17 on the bottom edge of the imager package 19. The imagerdie 13 is glued to the imager package 19 at the imager die 13 to packagebond locations 15.

[0033] During the assembly process the image sensor 12 is bonded to thecamera mounting plate 20 in a predetermined orientation using bondingmeans 21. This bonding is preferably performed with epoxy. The imagerfocus position can then be preferably tested with the method andapparatus of this invention before curing the epoxy. If the imagerposition meets predetermined specifications the epoxy will then becured. If the position does not meet predetermined specifications, theposition will be adjusted before curing the epoxy.

[0034]FIG. 3 shows a schematic of the measurement apparatus 41 includingan optical interferometer 80, a computer 90, with A-D converters anddata acquisition and control capability for passing interferometercontrol parameters and collecting interferometric data through datatransmission cables 66 from the interferometer 80 to the dataacquisition boards in computer 90, an optical multiplexer 60 and animager mounting apparatus 40. The imager mounting apparatus 40 includesa primary base 42 which preferably sits on any table, main verticalstandoffs 44 which fasten to the primary base 42 and base adapter 46, anoptical probe 48 mounted to the base adapter 46; a camera body mountingequivalent 50 having equivalent alignment features to a camera bodyincluding an imager mounting reference surface 52 attached to the baseadapter 46, with alignment holes 54 to receive alignment pins 24 fromthe imager mounting plate 20. For a film camera the camera body mountingequivalent 50 is designed to mimic the film rails in the camera. Theoptical probe 48 also includes individual optical probe chucks 53 and anoptical probe pellicle reference plane 55. A plurality of holes 56 arealso included in the camera body mounting equivalent 50 to match thelocations of the threaded screw holes 36 in a camera to align withalignment holes 26 in the imager mounting plate 20 of imager assembly10. FIG. 3 also shows a plurality of toggle clamps 51 and clamp bases 57that are attached to the base adapter 46 and are used to secure theimager assembly 10. The imager assembly 10 is mounted in the imagermounting apparatus 40 in place during the measurement.

[0035] During a measurement, light from a low-coherence source (notshown) inside the interferometer 80 is sent to the optical multiplexer60 by interferometer single mode fiber cable 64. The optical multiplexeris used to switch between different measurement locations on the imagersurface. This is done by switching the optical connection inside themultiplexer 60 between the various single mode optical fibers 62attached to the back of optical multiplexer 60 which are coupled to theindividual optical probe chucks 53 of optical probe 48 which define theindividual measurement locations on the surface of the imager 12. Duringa measurement sequence each of the optical probe chuck locations 53 aremeasured and analyzed in a defined sequence.

[0036] The optical probe 48 is defined as having a pellicle referenceplane 55. The preferred pellicle reference plane 55 is the surface of athick, glass, optical flat that faces the imager assembly 10 whenmounted in the imager mounting apparatus 40.

[0037] Referring to FIG. 4A, in one embodiment, a removable clamp tableassembly 77 is preferably used to secure the imager assembly 10 to theimager mounting apparatus 40 utilizing the plurality of toggle clamps 51(see FIG. 3). The removable clamp table assembly 77 includes a table top71, a plurality of clamp table standoffs 73 each with its own standoffalignment pin 75. Standoff alignment pins 75 are located at points tocorrespond with the imager mounting holes 54 in the camera mountingplate 20 of the imager assembly 10 used for mounting in the camera 30.

[0038]FIG. 4B shows a cross-section view of the imager assembly 10geometry when mounted for measurement purposes in the imager mountingapparatus 40. The cross-section view is indicated by the dashed line inFIG. 4A and labeled 4B. The optical probe 48 is on the bottom facing uplooking through the base adapter 46, and the imager assembly 10 isplaced face down in the camera body mounting equivalent 50 (shown inFIG. 3), and positioned on the top surface of the base adapter 46, sothat alignment pins 24 of the imager assembly 10 fit into alignmentholes 54 of the camera body mounting equivalent 50, mounting holes 26 inthe imager assembly 10 are aligned with the holes 56 of the camera bodymounting equivalent 50. When the alignment is complete the active areaof the image sensor 12 faces the optical probe 48. The clamp table 77 isthen positioned on top of the imager assembly 10 so that the standoffalignment pins 75 fit in the mounting holes 26 of the imager assembly10. The toggle clamps 51 are subsequently toggled to their contactposition so that the clamp load is distributed over the imager assembly10 at the positions of the mounting holes 26. A clamping force isapplied which mimics the loading that the imager assembly 10 would havewhen screws are inserted into the mounting holes and threaded into acamera body 30.

[0039]FIGS. 5A and 5B show a schematic of the measurement geometry andthe parameters measured with an interferometric based measurementsystem. FIG. 5A shows the measurement of the image sensor 12 while FIG.5B shows the measurement of a reference plate used to calibrate themeasurement system. Shown in FIGS. 5A and 5B are the optical probe 48,an optical probe chuck 53, the pellicle reference surface 55, and theimager mounting reference plane 52. FIG. 5A also shows the locations ofthe relevant imager assembly 10 components, including the imager die 13with imager plane 14 and the imager cover glass 16, with front surface25 and back surface 11 and the image sensor camera mounting referenceplane 22. When the imager assembly 10 is mounted into the imagermounting apparatus 40 the imager mounting reference plane 52 and theimage sensor camera mounting reference plane 22 are coincident in space.The distance PG is defined as the distance from the pellicle referenceplane 55 to the front surface 25 of the imager cover glass 16. Thedistance ‘g’ is defined as the distance between the imager plane 14 andthe back surface 11 of the imager cover glass 16. The thickness of theimager cover glass is defined as ‘t’. During a measurement, the opticalthickness of the imager cover glass (nt) is measured with theinterferometer 80, where ‘n’ is the group index of refraction of theimager cover glass 16.

[0040] In order to locate the height of the imager plane with respect tothe image sensor camera mounting reference plane 22, the distance fromthe pellicle reference plane 55 to the imager mounting reference plane52 in the imager mounting apparatus is measured, since these two planesare coincident during the measurement. The measurement is performed bymounting a flat reference plate 72 with flat reference plane 74 as shownin FIG. 5B onto the imager mounting reference plane 52. The flatreference plane 74 is coincident with the imager mounting referenceplane 52 and the image sensor camera mounting reference plane 22. Thedistance between the optical probe pellicle reference plane 55 and theflat reference plane 74 is defined as PP′ which is equivalent to thedistance between the pellicle reference plane 55 and the imager mountingreference plane 52. Thus a measurement performed using the flatreference plate 72 is used as a calibration to determine the parametersPP′ for each of the optical probe chuck locations.

[0041] The objective of the measurement is to determine the position ofthe imager plane 14 with respect to the image sensor mounting plane 22.Comparing this to specification limits for focus, when mounted inside acamera 30, a determination can be made if the camera 30 will be in focuswhen the imager assembly 10 is mounted inside the camera 30. Inperforming the calculation it is desired to measure the effectiveoptical distance between the imager plane 14 and the imager sensormounting plane 22 which we call the die-to-plate distance (DP). Thepresence of the imager cover glass increases the effective focaldistance of a lens by an amount Δ_(G) given by the relationshipΔ_(G)=t(1−1/n) where t is the thickness of the imager cover glass (16)and n is the group index of refraction of the cover glass at thewavelength of the light source used in the interferometer. The physicaldie to plate distance (DP)_(P) is given by

(DP)_(P) =PG+g+t−PP′.   (1)

[0042] In the digital camera application we are interested in theeffective optical die to plate distance DP which is equal to

DP=(DP)_(P) +Δ _(G) =PG+g+(nt)/(n)² −PP′  (2)

[0043] where PG is the distance from the pellicle reference plane 55 thefront surface 25 of the imager cover glass 16, ‘g’ is the gap betweenthe imager plane 14 and the back surface 11 of the imager cover glass16, ‘n’ is the group index of refraction of the imager cover glass, ‘t’is the thickness of the imager cover glass, Δ_(G) is the focus distanceincrease due to the presence of the imager cover glass 16 and PP′ is thedistance between the pellicle reference plane 55 and the imager mountingreference surface 52 at the probe chuck 53 measurement location. PP′ ismeasured by installing a flat reference plate 72 at the measurementlocation. The flat reference plate preferably includes a plurality ofholes to mate to the alignment pins 75 of the removable clamp tableassembly 71.

[0044] Note that no externally measured parameters are required in orderto determine the die to plate spacing. This is a drastic improvementcompared to the internal camera measurements made in the prior art whichrequire an external measurement such as a CMM measurement of a referencecradle to provide a reference distance required for determining imagerfocus error in a camera or film rail locations.

[0045]FIG. 6 shows an example of raw interferometer measurement dataobtained when measuring an imager assembly mounted into the imagermounting apparatus. During the measurement the interferometer is made toscan a distance large enough to measure the relevant distances g, nt andPG. The data is obtained using an interferometer operating in anautocorrelation geometry. An example of an interferometer operating inan autocorrelation geometry is shown in FIG. 11 of U.S. Pat. No.5,757,486 referenced above. The interferometer continually scans backand forth a distance greater than the largest measured distance PG andis made to cross the zero-crossing point in the interferometer, theposition at which the path lengths of the 2 arms of the Michelsoninterferometer are equal in length. Motor scan reversal points are shownin FIG. 6 as the curved lines. Peaks 110, 120, 210 and 220 are zerocrossings of the interferometer and all measured distances arereferenced to the nearest adjacent zero crossings. The interferometertrace segment shown in FIG. 6 correspond to one complete interferometermotor scan cycle plus passing again across the zero crossing points. Theinterferometer motor reverses scan directions between pairs of peaks 110and 120, 160 and 170 and 210 and 220.

[0046] The imager gap ‘g’ is determined by measuring the distancebetween peaks 120 and 130 and/or 200 and 210, the optical thickness ntof the imager cover glass 16 is determined by measuring the distancebetween peaks 120 and 140 and and/or 190 and 210, and the pellicle gap(PG) is determined by measuring the distance between peaks 120 and 160and/or 170 and 210. The distance between peaks 120 and 150 and or peaks180 and 210 define the distance g+nt. During a measurement usually a setinterval of time, such as, 1 second is used to repetitively scan theinterferometer back and forth at a typical rate such as 10 Hz. Thisallows 20 measurements per second, and an average value of themeasurements would be stored in a computer file. Interferometer peaklocations are determined by the methods of the references. Suitable peaklocation calculation procedures are described in U.S. Pat. Nos.5,596,409 and 5,659,392 referenced above.

PARTS LIST

[0047] 10 imager assembly

[0048] 11 imager cover glass back surface

[0049] 12 image sensor

[0050] 13 imager die

[0051] 14 imager plane

[0052] 15 imager die to package bond locations

[0053] 16 imager cover glass

[0054] 17 imager electrical connections

[0055] 18 imager gap

[0056] 19 imager package

[0057] 20 camera mounting plate

[0058] 21 bonding means

[0059] 22 image sensor camera mounting reference plane

[0060] 23 cover glass bond perimeter

[0061] 24 alignment pins

[0062] 25 imager cover glass front surface

[0063] 26 mounting holes

[0064] 28 image sensor cutout

[0065] 30 camera body

[0066] 31 lens flange-mounting ring

[0067] 32 camera reference plane

[0068] 34 camera alignment pin receiver holes

[0069] 35 camera mounting reference structure

[0070] 36 camera threaded holes

[0071] 40 imager mounting apparatus

[0072] 41 measurement apparatus

[0073] 42 primary base

[0074] 44 main vertical standoff

[0075] 46 base adapter

[0076] 48 optical probe

[0077] 50 camera body mounting equivalent

[0078] 51 toggle clamps

[0079] 52 imager mounting reference surface

[0080] 53 optical probe chucks

[0081] 54 alignment holes

[0082] 55 optical probe pellicle reference plane

[0083] 56 holes

[0084] 57 clamp base

[0085] 60 optical multiplexer

[0086] 62 single mode optical fibers

[0087] 64 interferometer single mode fiber cable

[0088] 66 data transmission cables

[0089] 71 table top

[0090] 72 flat reference plate

[0091] 73 clamp table standoff

[0092] 74 flat reference plane

[0093] 75 standoff alignment pins

[0094] 7 clamp table assembly

[0095] 80 optical interferometer

[0096] 90 computer

[0097] 110 peaks

[0098] 120 peaks

[0099] 130 peaks

[0100] 140 peaks

[0101] 150 peaks

[0102] 160 peaks

[0103] 170 peaks

[0104] 180 peaks

[0105] 190 peaks

[0106] 200 peaks

[0107] 210 peaks

[0108] 220 peaks

What is claimed is:
 1. A method for determining a position of an imager plane relative to an image sensor camera-mounting plane in an imager assembly, comprising the steps of: a) temporarily mounting the imager assembly onto an imager mounting apparatus having an imager mounting reference surface such that the imager sensor camera-mounting reference plane and the imager mounting reference surface are in intimate contact; wherein the imager mounting apparatus includes an optical probe with a pellicle reference surface in a predetermined orientation with respect to the imager mounting reference surface such that the pellicle reference surface is disposed at a first depth relative to the imager mounting reference surface; b) utilizing low-coherence light interferometry to determine (i) a second depth from the pellicle reference surface to a front surface of the optically transparent plate, (ii) an optical thickness of the imager cover glass, and (iii) a third depth from a back surface of the imager cover glass to the imager plane; and c) calculating the optical position of the imager plane relative to the imager sensor camera-mounting reference plane.
 2. The method claimed in claim 1, wherein the step of calculating the optical position of the imager plane relative to the imager sensor camera-mounting reference plane DP, defined by the relationship DP=PG+g+(nt)/(n)²−PP′, wherein the first depth is defined as PP′, wherein the second depth is defined as PG, and wherein the third depth is defined as g, and the optical thickness of the imager cover glass is defined as (nt), where ‘n’ is an index of refraction of the imager cover glass. 